The determination of the moisture content of tobacco leaf is important to the tobacco processing industry. Frequently, the moisture content of tobacco leaf is measured with a reflectance type infrared analyzer. The instrument quantitatively measures the water content of solids by measuring the infrared reflectance of the sample to be analyzed in response to two infrared wavelengths, one within one of the water infrared absorption bands, and the second beam just outside of the water absorption band. The difference in the reflectance provides a measure of the moisture content of the sample. Water absorbs radiation in the near infrared region of the spectrum, which may be defined as the wavelengths extending between 0.8-2.5 microns. Characteristic absorptions for water are found at wavelengths of 1.9 and 1.4 microns. Accordingly, suitable reference wavelengths may be, for example, 1.6 and 1.2 microns.
In operation, the instrument must be empirically calibrated using samples of known moisture content. Typically, the instrument is provided with adjustments for the regulation of the "zero" point of the instrument and the "range" of moisture which will cause a full scale deflection. For calibration purposes, two or more samples of differing moisture contents generally covering the range of moisture content to be analyzed are tested. The zero point and range controls are adjusted so that meter readings for a normal moisture level will fall in a mid scale region. The range is adjusted so that the meter range will encompass the various moisture levels to be measured.
Because of drift in the characteristics of the instrument's meter circuitry, infrared moisture analyzers must be recalibrated periodically to vertify that the low and high meter readings have not changed in response to a pre-determined absorption levels. For this purpose, a double-sided black-white disc is usually supplied by the manufacturer. The disc is intended to provide a reproducible low reflectance and high reflectance calibration standard level which will be used for the purpose of recalibrating the instrument, if needed.
The commercially supplied discs, however, have been found to be unsatisfactory. In part, this is because the reflectivity of the discs in response to the infrared radiation employed in the instrument is not related to the reflectivity of the disc in response to visible light. Indeed, and surprisingly, it has been found that the reflectivity difference between the black and the white side of one commercial calibration disc in response to infrared radiation is negligible. Moreover, reflection from the disc is typically composed of both specular and diffuse reflection. In the commercial calibration disc, specular reflection is substantial. Specular reflection is critically dependent on the angular adjustment of the calibration disc relative to the optical path of the instrument. Where small differences in reflectivity must be calibrated, the use of the black-white disc results in readings which are not reproducible.